Java LanguageJava plugin system implementations

Remarks

If you use an IDE and/or build system, it is much easier to set up this kind of project. You create a main application module, then API module, then create a plugin module and make it dependent on the API module or both. Next, you configure where the project artifacts are to be put - in our case the compiled plugin jars can be sent straight to 'plugins' directory, thus avoiding doing manual movement.

Using URLClassLoader

There are several ways to implement a plugin system for a Java application. One of the simplest is to use URLClassLoader. The following example will involve a bit of JavaFX code.

Suppose we have a module of a main application. This module is supposed to load plugins in form of Jars from 'plugins' folder. Initial code:

package main;

public class MainApplication extends Application
{
    @Override
    public void start(Stage primaryStage) throws Exception
    {
        File pluginDirectory=new File("plugins"); //arbitrary directory
        if(!pluginDirectory.exists())pluginDirectory.mkdir();
        VBox loadedPlugins=new VBox(6); //a container to show the visual info later
        Rectangle2D screenbounds=Screen.getPrimary().getVisualBounds();
        Scene scene=new Scene(loadedPlugins,screenbounds.getWidth()/2,screenbounds.getHeight()/2);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
    public static void main(String[] a)
    {
            launch(a);
    }
}

Then, we create an interface which will represent a plugin.

package main;

public interface Plugin
{
    default void initialize()
    {
        System.out.println("Initialized "+this.getClass().getName());
    }
    default String name(){return getClass().getSimpleName();}
}

We want to load classes which implement this interface, so first we need to filter files which have a '.jar' extension:

File[] files=pluginDirectory.listFiles((dir, name) -> name.endsWith(".jar"));

If there are any files, we need to create collections of URLs and class names:

    if(files!=null && files.length>0)
    {
        ArrayList<String> classes=new ArrayList<>();
        ArrayList<URL> urls=new ArrayList<>(files.length);
        for(File file:files)
        {
            JarFile jar=new JarFile(file);
            jar.stream().forEach(jarEntry -> {
                if(jarEntry.getName().endsWith(".class"))
                {
                    classes.add(jarEntry.getName());
                }
            });
            URL url=file.toURI().toURL();
            urls.add(url);
        }
        
    }

Let's add a static HashSet to MainApplication which will hold loaded plugins:

static HashSet<Plugin> plugins=new HashSet<>();

Next, we instantiate a URLClassLoader, and iterate over class names, instantiating classes which implement Plugin interface:

URLClassLoader urlClassLoader=new URLClassLoader(urls.toArray(new URL[urls.size()]));
classes.forEach(className->{
    try
    {
        Class cls=urlClassLoader.loadClass(className.replaceAll("/",".").replace(".class","")); //transforming to binary name
        Class[] interfaces=cls.getInterfaces();
        for(Class intface:interfaces)
        {
            if(intface.equals(Plugin.class)) //checking presence of Plugin interface
            {
                Plugin plugin=(Plugin) cls.newInstance(); //instantiating the Plugin
                plugins.add(plugin);
                break;
            }
        }
    }
    catch (Exception e){e.printStackTrace();}
});

Then, we can call plugin's methods, for example, to initialize them:

if(!plugins.isEmpty())loadedPlugins.getChildren().add(new Label("Loaded plugins:"));
    plugins.forEach(plugin -> {
        plugin.initialize();
        loadedPlugins.getChildren().add(new Label(plugin.name()));
});

The final code of MainApplication:

package main;
public class MainApplication extends Application
{
    static HashSet<Plugin> plugins=new HashSet<>();
    @Override
    public void start(Stage primaryStage) throws Exception
    {
        File pluginDirectory=new File("plugins");
        if(!pluginDirectory.exists())pluginDirectory.mkdir();
        File[] files=pluginDirectory.listFiles((dir, name) -> name.endsWith(".jar"));
        VBox loadedPlugins=new VBox(6);
        loadedPlugins.setAlignment(Pos.CENTER);
        if(files!=null && files.length>0)
        {
            ArrayList<String> classes=new ArrayList<>();
            ArrayList<URL> urls=new ArrayList<>(files.length);
            for(File file:files)
            {
                JarFile jar=new JarFile(file);
                jar.stream().forEach(jarEntry -> {
                    if(jarEntry.getName().endsWith(".class"))
                    {
                        classes.add(jarEntry.getName());
                    }
                });
                URL url=file.toURI().toURL();
                urls.add(url);
            }
            URLClassLoader urlClassLoader=new URLClassLoader(urls.toArray(new URL[urls.size()]));
            classes.forEach(className->{
                try
                {
                    Class cls=urlClassLoader.loadClass(className.replaceAll("/",".").replace(".class",""));
                    Class[] interfaces=cls.getInterfaces();
                    for(Class intface:interfaces)
                    {
                        if(intface.equals(Plugin.class))
                        {
                            Plugin plugin=(Plugin) cls.newInstance();
                            plugins.add(plugin);
                            break;
                        }
                    }
                }
                catch (Exception e){e.printStackTrace();}
            });
            if(!plugins.isEmpty())loadedPlugins.getChildren().add(new Label("Loaded plugins:"));
            plugins.forEach(plugin -> {
                plugin.initialize();
                loadedPlugins.getChildren().add(new Label(plugin.name()));
            });
        }
        Rectangle2D screenbounds=Screen.getPrimary().getVisualBounds();
        Scene scene=new Scene(loadedPlugins,screenbounds.getWidth()/2,screenbounds.getHeight()/2);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
    public static void main(String[] a)
    {
            launch(a);
    }
}

Let's create two plugins. Obviously, the plugin's source should be in a separate module.

package plugins;

import main.Plugin;

public class FirstPlugin implements Plugin
{
    //this plugin has default behaviour
}

Second plugin:

package plugins;

import main.Plugin;

public class AnotherPlugin implements Plugin
{
    @Override
    public void initialize() //overrided to show user's home directory
    {
        System.out.println("User home directory: "+System.getProperty("user.home"));
    }
}

These plugins have to be packaged into standard Jars - this process depends on your IDE or other tools.

When Jars will be put into 'plugins' directly, MainApplication will detect them and instantiate appropriate classes.